Can I build an Accessory Dwelling Unit (ADU) on my property?
  • Probably! Accessory dwelling units can be either attached to a primary residence or detached as a separate structure. Classic examples are a dwelling above a garage, or a cottage for rental income. ADU’s require planning approval and a residential building permit. They can be manufactured or modular homes.  
  • The maximum size of an ADU is 1,200 square feet.
  • In areas classified as Urban and Suburban in the Comprehensive Plan, you may choose to build up to two ADU’s – one attached and one detached. In all other areas of the county, one ADU is allowed.
  • There is no restriction on owner-occupancy of the primary residence or the ADU – one or both can be occupied by tenants.
  • If you are using a domestic well and/or onsite septic system for the ADU, you will need to talk with the Environmental Health Unit about their requirements. Their email is and phone number is 360-414-5599 - press 5 and option 2.

Show All Answers

1. What are the first steps in planning my project?
2. What are my setbacks?
3. Can I meet with County staff to discuss my project?
4. How do I get an address for my property?
5. Can I build an Accessory Dwelling Unit (ADU) on my property?
6. Does Cowlitz County charge "impact fees" for schools, parks, transportation, etc., for new development?
7. Do I need to manage stormwater from my project?